1. Field of the Invention
The invention relates generally to an optical wavelength converter. In particular, the invention relates to an optical wavelength converter incorporated into an optical switching element.
2. Background Art
Converting the wavelength of a light signal has always presented a challenge, particularly in optical communications systems in which an optical carrier is modulated with a data signal and it is desired to convert the wavelength of the optical carrier while maintaining the data signal without the need to convert to an intermediate electrical signal corresponding to the data.
Wavelength conversion has been suggested for greatly increasing the capacity and flexibility of switched optical networks. Wavelength division multiplexing (WDM) has long been known in which, for example, a silica optical fiber carries a number of discrete optical carriers in the 1525 to 1575 nm band, each impressed with its separate data signal. Electronic data signals are presently limited to about 10 to 40 gigabits per second (Gbs), but if these data rates are multiplied by the number W of WDM channels, where W is reaching 80 and higher, the total data capacity of a single fiber exceeds a terabit per second (Tbs). Such high data rates are anticipated to be needed as more visual forms of data begin to dominate the communications networks.
However, modem communication systems are based upon a complex network connected at its edges to users with typically many switching nodes separating a pair of users. A primary example is the Internet based upon the Internet Protocol (IP). Signals need to routed through the nodes of the network, which requires routers at the nodes to perform such selective routing. Heretofore, routers have been based on electronic switches, typically crossbars. As a result, WDM signals need to be optically demultiplexed, electrically detected, electrically switched, impressed upon respective optical carriers, and optically multiplexed before the components signals are sent in their respective proper directions. That is, regeneration of the optical signals needs to be performed with currently available routers. This design does not scale well with vastly increased optical channels.
All-optical WDM communication networks have been proposed in which wavelength cross connects (WXCs) located at nodes in the network redirect the separate WDM signals according to their wavelength. However, conventional WXCs require switching times that are much longer than the duration of packets typical of IP networks and other flexible communications networks. Further, WXCs maintain the carrier wavelength of the data signal so that using a wavelength identifier to route a signal between pairs of a number of users presents a significant network management problem for large networks in reusing wavelengths in different parts of the network.
Many of these problems can be overcome by the use of wavelength converters. In xe2x80x9cWavelength conversion technologies for WDM network applications, Journal of Lightwave Technology, vol 14, no. 6, June 1996, pp. 955-966, I have described some of the network applications for wavelength converters and different ways of implementing them. In U.S. Provisional Application, Ser. No. 60/185,640, filed Feb. 29, 2000 and in U.S. patent application Ser. No. 09/654,384, filed Sep. 1, 2000, I describe an optical router implemented with wavelength converters capable of throughput capacity of petabits per second (1015 bits per second). This patent application is incorporated herein by reference in its entirety.
A switching fabric 10 illustrated in FIG. 1 can be used in such a high-speed optical router in which WDM signals on K input optical channels 12, typically optical fibers, are switched to any of K output optical channels 14. Optical demultiplexers 16 associated with each of the input fibers 12 separate the optical WDM signal into its W wavelength components. Input wavelength converters 18 convert the wavelength of the optical carriers on each of the demultiplexers outputs to a selected one of a number of wavelengths before these signals are input to a WKxc3x97WK wavelength router 20. Such a wavelength router 20 may be implemented as one or more array waveguide gratings (AWGs) and may be a passive device in which the switching direction of an input signal from any input port to any output port is determined completely by the carrier wavelength of the input signal as impressed by the input wavelength converters 18. Note that the size of the wavelength router 20 may be reduced if some limitations are imposed on the number of allowed switching paths. The output ports of the wavelength router 20 are connected to respective output wavelength converters 22 which freely convert the carrier wavelengths to new values determined by the WDM wavelength comb of the output channels 14. Optical multiplexers 24 receive W wavelength carriers and combines them onto associated optical output channels 14.
In order to achieve the capacity and flexibility of the optical router described in the aforecited patent application, the wavelength converters 18, 22 must be able to switch between any of the WDM wavelengths and should be able to do this in a time not much greater than the duration of the packet length, which in a typical design is a random length, for example 48 bytes or 1.5 kilobytes. Some signal delay is tolerated at the nodes, but it should be minimized for low signal latency on the network.
One type of wavelength converter applicable to optical switching is a gated wavelength converter, which may be implemented in a Mach-Zehnder interferometer converter 30, described briefly in my technical article cited above and illustrated in schematic plan view in FIG. 2. The converter 30 is integrated on, for example, an InP opto-electronic chip 32 formed with a Mach-Zehnder interferometer 34 having optical waveguides formed into two arms 36, 38. Forward biased optical semiconductor amplifiers (SOAs) are formed in active region 40, 42 of the both arms 36, 38. Forward biasing means that a positive voltage +V is applied to the p-type side of the semiconductor diode relative to the n-type side, which is typically grounded. All the illustrated waveguide, including that in the active regions 40, 42 is single-mode through the band of the optical carriers. An optical data signal at carrier wavelength xcex1 is input to a waveguide coupler at one end of the upper arm 36 configured to supply the data signal only to the upper arm 36. An unmodulated probe signal at wavelength xcex2 is input to another coupler at the other end of the upper arm 36. Note that the probe signal counter-propagates relative to the xcex1 data signal. That coupler divides the probe signal between the upper and lower arms 36, 38. The probe signal, after propagating through the two arms 36, 38 is recombined by the optical coupler on the left to an output waveguide 44.
The optical amplifiers in both arms 36, 38 are operated near or in optical saturation so that when the power of the on-state of the xcex1 optical data signal passes through the upper active region 40, it causes a change of phase relative to the lower active region 42, which does not experience the extra xcex1 signal power. In the off-state of the xcex1 optical data signal, there is no phase difference between the two active regions 40, 42. Sometimes differential biasing imposes a time-invariant phase difference between the two arms 40, 42. As a result of the induced temporally varying phase difference, the xcex2 probe signal will experience a phase differential in the two Mach-Zehnder arms 34, 36 depending upon the modulation state of the xcex1 optical data signal. The waveguide coupler at the end of the Mach-Zehnder interferometer opposite the xcex2 probe source receives both xcex2 signals and combines them onto the output waveguide 44. Depending upon the relative phase differences between the two arms, the two signals interference either constructively or destructively, thereby producing on the output waveguide 44 a xcex2 optical data signal of carrier frequency xcex2 modulated according the data signal originally carried on the xcex1 carrier.
This type of Mach-Zehnder wavelength converter, however, suffers several disadvantages. It consumes excessive amounts of electrical power, requiring about 300 mA of amplifier biasing current, approximately 500 mW of power. In view of the large number of wavelength converters required for a useful optical router, the large power presents powering problems and precludes integration of a significant number of converters on a single opto-electronic chip. Furthermore, the data rate of this type of wavelength converter is limited by the dynamics of the semiconductor amplifier, typically having a response time of 0.1 to 1 ns in typical InP opto-electronics. Accordingly, 10 Gbs single-channel data rates are at best marginal, and 40 Gbs rates are infeasible unless very high optical power of the order of 100 mW is used for the probe signal of the second wavelength.
Accordingly, an optical wavelength converter is needed which can convert carrier wavelengths while preserving their modulation and which converter consumes reduced levels of power. It is also desired that such a converter allow increased modulation rates of the converted optical signal.
An optical wavelength converter includes an interferometer having a semiconductor junction that is reversed biased. A first optical signal at a first wavelength is modulated according to a data signal and passes through the reversed biased junction as an unmodulated second optical signal at a second wavelength passes therethrough. The resultant second optical signal is combined with the unmodulated second optical signal to produce a second optical signal modulated according to said data signal. Preferably, the first and second optical signals counter-propagate through the reversed biased junction.
Preferably, the semiconductor junction has an optical band gap of a wavelength shorter than the first and second wavelengths. Preferably also, the semiconductor junction is reversed biased into avalanche with a multiplication factor, for example, of at least 10.
In one embodiment, the interferometer is a Mach-Zehnder interferometer having two arms, each with such a reversed biased semiconductor junction. The first optical signal passes through only one arm, and the second optical signal is split between both arms. The second optical signal is then recombined.
A semiconductor optical amplifier may amplify the first optical signal before it enters the interferometer.
A tunable diode laser may provide the second optical signal.
Either or both of the optical amplifier and the tunable diode laser may be integrated on the same opto-electronic chip as the interferometer and semiconductor junction. A plurality of sets of such elements may be integrated on the chip.
The optical input signal may first be converted to an intermediate signal at a fixed third wavelength which is then passed into the interferometer. Preferably, the third wavelength is shorter than either wavelength of the first and second optical signals. The initial wavelength conversion may be performed by cross-gain modulation.
Preferably, the interferometer is integrated on the same opto-electronic chip with some or all of the above described elements.